The development of fiber-reinforced polymeric composite materials has been of great benefit as these materials can provide excellent strength characteristics and corrosion resistance at low densities. These materials have been of particular benefit in the transportation industry, where the ability to produce light weight, strong, and resistant laminate panels has greatly enhanced efficiency of vehicles and decreased both construction and operating costs.
Laminate fiber reinforced polymeric composites are not without issues, however. For instance, material characteristics that can be highly beneficial in certain aspects, such as stiffness, can cause manufacturing and operational concerns. By way of example, impact events produced by operating conditions (e.g. impacts with foreign objects, weather events, etc.) and human errors (tool drops, ground collisions, etc.) can cause delamination and cracking in composite structures. Such concerns have led to the development of structural health monitoring (SHM) systems that have been incorporated with traditional maintenance procedures, particularly in vehicles such as aircraft, in which such damage can be easily overlooked in visual inspection but in severe operating conditions can lead to catastrophic failure and even loss of life in extreme cases.
SHM is used to determine the condition of a structure by use of sensors that are embedded in or otherwise attached to the structure. SHM is generally performed with either a passive or an active system. Passive SHM includes monitoring one or more of a number of parameters including, but not limited to, loading stress, environment action, performance indicators, and acoustic emission (for instance from cracks). Passive SHM uses sensors that “listen” but do not interact with the structure. As such, the passive method does not generally provide direct measurement of damage presence and size, but the general health state of the structure can be inferred from analysis of the response of passive sensors.
Active SHM utilizes proactive interrogation of sensors embedded in/on the structure to detect damage extent and thereby determine a more detailed analysis. Methods used for active SHM resemble those of nondestructive evaluation (NDE), e.g., ultrasonics, eddy currents, etc., with one difference being that the active SHM methods are carried out with permanently affixed sensors.
One widely used active SHM method employs piezoelectric wafer active sensors (PWAS) to determine the presence of cracks, delamination, disbonds, corrosion, etc. in composite polymer laminates. Due to similarities to NDE ultrasonics, this approach is also known as embedded ultrasonics.
While the current SHM systems have provided improvement to the art, room for further improvement exists. For instance, while passive SHM systems can provide general information with regard to possible damage-causing events, specific knowledge about the damage, including confirmed existence of the damage, is sometimes not available. Moreover, to obtain as much information as possible with a passive system, a large number of sensors may be needed, which requires a great deal of data acquisition components, processing hardware, etc. A system that includes a large number of components can take up a great deal of space and increase the weight of the structure. An active SHM system can provide greater detail with regard to location and extent of damage, but it also requires heavy and complex data acquisition and processing components, particularly when contemplating continuous monitoring of the structure, further adding to the weight and space requirements of a system.
What are needed in the art are SHM systems and methods that can continuously monitor a structure while providing information with regard to location and severity of damage due to, e.g., impact events. Moreover, a system that can provide improved monitoring outcome with minimal added weight and space requirements to the supporting structure would be of great benefit.